This invention relates to the epitaxial growth of semiconductor materials. Epitaxy is the process of forming a highly controlled crystalline layer on top of a crystalline substrate or wafer. The crystal structure for the epitaxial layer is usually identical or similar to that of the substrate, however it may contain different intentionally introduced impurities or be of a different material composition. Epitaxy is used in the creation of many types of electronic devices, including laser diodes, high mobility transistors, and optoelectronic integrated circuits.
There are presently several techniques used to grow epitaxial films. The most common approaches are by physical deposition (such as molecular beam epitaxy), chemical vapor deposition, or liquid phase epitaxy. This invention pertains to chemical vapor deposition (CVD), whereby the epitaxial layers are formed by the thermal decomposition (pyrolysis) of reactant gases above a heated substrate. The system is usually a cold-wall reactor, whereby only the substrate is heated and the reaction chamber walls are unheated. Heating of the substrate is usually accomplished by thermal contact with a graphite susceptor, which is heated using an infrared or ultraviolet light source, a proximity resistive heater, or an inductive radio frequency (RF) system.
The pyrolysis process takes place within a reaction chamber. A carrier gas, usually ultrapure hydrogen, containing carefully controlled amounts of the reactant gases, flows through this chamber and over the heated substrate. The transport of the reactants over the substrate and their resulting thermochemical reactions makes CVD a very complex process where small changes can have large effects. Hence, the design of the reaction chamber is critical for successful, controlled, uniform deposition.
Once the reactant gases start to thermally decompose over the substrate, the amount of unreacted gas is reduced. The carrier gas downstream of the susceptor will have less constituent reactants than that near the upstream edge of the heated substrate. This effect is known as reactant depletion. In order to provide uniform gas composition across the substrate, the gas flow must be laminar and some method to compensate for reactant depletion must be used. Again, the design of the reaction chamber is critical to assure a uniform, laminar gas flow.
The design of the gas handling section is another critical aspect of the reactor system. In order to achieve thin epitaxial layers such as quantum wells, it is necessary to have a short, but controlled, transit time of the reactants over the substrate. It is thus necessary to have both abrupt switching of the reactants into the growth chamber and also high gas velocities within the reaction chamber. The abrupt switching is accomplished by a run-vent manifold system that switches the reactants between the reaction chamber and a vent line that bypasses the reactor chamber. The vent line is reconnected to the exhaust system downstream. Both the reaction chamber and vent line must be pressure-balanced to prevent unnecessary transients to the gas stream. Higher gas velocities are accomplished by maintaining the reaction chamber and exhaust system at sub-atmospheric pressures, typically between 50 and 100 Torr.
There are three general types of reaction chamber geometries used for chemical vapor deposition, which are delineated by the direction of gas flow relative to the substrate surface. In horizontal systems, the wafer is placed on top of a boat or a susceptor with its face is parallel to the gas stream or slightly angled to prevent reactant depletion. Conventional horizontal reactors are linear. The gas is injected from the manifold at one end, passed over the substrate in the middle and removed by the exhaust system at the other end. In vertical systems, the gas flow is perpendicular to the surface of the substrate. Again, the gas flow is linear. It is injected at the top of the chamber and collected at the bottom. The third type of system is the barrel which is a hybrid of the first two. It is generally used for large scale wafer growth. The substrates are placed on a cylindrical or conical shaped barrel, and the reactant gases are injected vertically from the top and collected at the bottom. The reaction chamber shape is very much like a vertical reactor, but the gas flow over the substrates is similar to the flow in a horizontal reactor. For all three systems, the susceptor area is usually rotated to provide more uniform gas and temperature distributions.
However, traditional multiple substrate reactors have uniformity problems if their reaction chamber design prevents the use of a viable rotation system. Even in systems that have substrate rotation, substrate-to-substrate nonuniformity exists and the growth characteristics will change with the number of substrates in the reactor.
Another traditional problem with CVD systems is that even though these systems are usually cold-wall reactors, some of the reactants are still deposited on the interior walls of the reaction chamber. Typical reaction chamber geometries do not allow for water-cooling the quartz reactor walls during deposition and prevent the use of an in-situ bakeout method to clean the inside of the reaction chamber between growth runs.